Extreme ultraviolet lithography, which uses a source at 13.5 nm wavelength, is a promising technology for 0.032 micron integrated circuit (IC) fabrication. Since the absorption at that wavelength is very strong in all materials, EUV lithography employs Si/Mo multilayer mirrors as reflective optics, rather than refractive optics used in longer wavelength (optical) lithography. The strong absorption requires the use of reflective mask reticles, rather than through-the-mask reticles used in longer wavelength lithography. The Si/Mo multilayer reflective masks are made by depositing highly detailed absorber patterns on top of the Si/Mo multilayer mirrors.
There are many issues to be resolved in order to realize EUV lithography, such as, developing a powerful EUV source, robust components that direct the radiation (mirrors), and robust components that define the integrated circuit features (reticles). An EUV source with a collectable radiation power of 50 W to 150 W at over 5 kHz in the spectral range of 13-14 nm is required to achieve requirements for high volume manufacturing of 300 mm wafers. Laser-induced and electrical discharge gas plasma devices (EUV lamps) are under investigation as promising EUV sources. These sources generate EUV radiation by heating certain materials into a plasma to such a level, in the many 100,000's C, that the material emits EUV radiation. Potential source materials which emit EUV radiation at excited energy levels include xenon, oxygen, and lithium.
Not withstanding the reflectivity of the Si/Mo multilayer mirrors and reflective masks, they remain subject to tremendous heat loads from absorption of the EUV radiation. The reflectivity of the mirrors and reflective masks degrades over time requiring periodic replacement. Improvements to increase the lifetime of the mirrors and reflective masks is desired.
Along with the radiation, the EUV sources are also emitters of high velocity particulate contamination. The high velocity particles are a potential source of harmful erosion of surfaces upon which they impinge, as well as a contamination source, as they deposit onto the reflective surfaces of the components upon which they reflect. The Si/Mo multilayer mirrors and reflective masks are highly sensitive to this erosion and particle contamination.
Optical lithography incorporates the use of pellicles to protect the reticle from contamination. A pellicle can not be used with EUV lithography due to the high heating of the pellicle from EUV absorption.
Debris shields are being investigated, through which the EUV radiation is passed to catch or filter the particles. But in the effort to maximize photon illumination, the “mesh” size has to be a compromise between particle pass-through rate and reduction in EUV power.
Until a more effective process is used to prevent particulate contamination, frequent cleaning of the Si/Mo multilayer mirrors and reflective masks will be required. But the delicate multilayer coatings used in EUV mirrors and reticles cannot withstand harsh or frequent cleaning. The cleaning chemicals used in the reticle cleaning process for conventional binary masks cannot be used as they etch the delicate surfaces. Improvements are needed to permit thorough cleaning of the Si/Mo multilayer mirrors grazing collector incidence mirror, and reflective masks without damage.
In order for EUV lithography to meet commercial requirements and demands, including reliability, productivity, and maintenance, lifetime-extending improvements are necessary for the Si/Mo multilayer mirrors and reflective masks.